The invention disclosed and claimed herein generally pertains to a method for reducing artifacts in acquired magnetic resonance (MR) images, wherein the artifacts result from translational motion of a patient or other object of imaging. More particularly, the invention pertains to a method of such type wherein an MR point source is employed to acquire data which may be used to determine and correct phase errors resulting from such motion. Even more particularly, the invention pertains to a method of such type wherein a reference MR signal, derived from the point source, is compared with a k-space signal representing the point source when it is moving in unison with the object of imaging.
In conventional MR imaging, the scan time typically lasts a few seconds to several minutes. During this time, physiologic motion (e.g., cardiac, respiratory, gastrointestinal, and vascular motion), as well as a patient's gross movements (both voluntary and involuntary) can contaminate the spatially encoded MR signals, causing ghosting and blurring artifacts. Some of these motions, such as cardiac and respiratory, are periodic in nature. Other motions, such as involuntary motion of a patient, or uncontrolled movements of small children, are non-periodic and thus tend to be random or unpredictable.
In a conventional imaging technique such as spin warp, k-space is sampled by a series of lines parallel to the frequency-encoding axis (k.sub.x -axis), with each line corresponding to a unique location along the phase-encoding axis (k.sub.y -axis). Typically, each k.sub.x -line is acquired with a single pulse sequence. The acquisition time per k-space line lasts only a few milliseconds. Motion during this short acquisition time, known as intra-view motion, is negligible and does not cause substantial image degradation. However, different k-space lines along the phase-encoding direction are acquired by repeating the pulse sequence with different phase-encoding gradients. The time span among these k-space lines can be hundreds of milliseconds or even seconds. Thus, the k-space data along the phase-encoding direction are particularly susceptible to motion. Such motion, referred to as view-to-view motion, can cause serious image artifacts.
In the past, a technique known as gating, wherein data acquisition is synchronized with motion, has been used to reduce motion-induced errors. Such technique is discussed, for example, by W. J. Rogers, Jr., and E. P. Shapiro in "Effect of RR interval variation on image quality in gated, two-dimensional, Fourier MR imaging", Radiology, vol. 186, pp. 883-887 (1993). However, the gating technique can only be used in connection with motion which is periodic. Moreover, such technique will significantly slow down data acquisition if the periods of successive motion cycles are comparatively long, e.g., on the order of seconds.
In another motion correction technique, known as navigator echo correction, an additional echo is acquired in the same pulse sequence that acquires the k-space data. This echo, referred to as a navigator echo, is used to determine the instantaneous position of the object when the sequence is played out, and is subsequently used to retrospectively correct the k-space data acquired by the same sequence, or to prospectively re-acquire the motion-contaminated k-space data if motion exceeds a pre-determined threshold. The navigator technique is described, for example, by R. L. Ehman and J. P. Felmlee, Radiology, vol. 173, pp. 255-263 (1989), and by Z. W. Fu, et al., Magn. Reson. Med., vol. 34, pp. 746-753 (1995). The use of navigator echoes always requires additional data which can lead to longer imaging times.
Deficiencies of the prior art cited above are addressed, at least in part, by means of the MR imaging technique disclosed in U.S. patent application Ser. No. 08/987,594, filed Dec. 9, 1997 by Xiaohong Zhou, one of the co-inventors herein. Such application is commonly assigned herewith, to the General Electric Company, and the technique thereof is known as MORKA (motion reduction by k-space alignment). In accordance with such technique, two (or more) additional k-space lines or signals are acquired in a direction orthogonal to the nominal k-space data. From the two additional k-space signals, two simultaneous equations are obtained, for use in calculating the translational spatial displacements .DELTA.x.sub.n and .DELTA.y.sub.n,, for each phase encoding step (or view) n. This technique has been found to work in certain cases, particularly where the two additional signals are acquired close to the center of k-space. However, when the size of the imaged object is comparable to the imaging field-of-view (FOV), the k-space imaging signal usually decays very fast. Away from the k-space center, therefore, the phase calculation is subject to noise perturbations, leading to erroneous results. Even when the signal-to-noise ratio (SNR) is relatively high, the k-space signal can have nodal points with zero amplitudes, making the phase uncertain. These problems have limited the use of the MORKA technique.
To date such techniques have mainly been discussed with respect to two dimensional (2D) imaging, in which case only the in-plane translational motion can be corrected. However the concepts invoked can also be applied to three dimensional (3D) imaging, whereby the full 3D motion can potentially be corrected.